Welfare implications of injuries and deformities in wild fish

Authored by Wild Animal Initiative Strategy Researcher Simon Eckerström Liedholm, this paper was published in April 2026 by Animal Welfare.

Abstract

External injuries and morphological deformities may serve as useful indicators when assessing the welfare of wild animals, as they can be easily observed, be scored in a non-disruptive manner, and likely correlate with reductions in welfare in many contexts. However, the welfare effects of injuries and deformities have so far been mostly examined for animals in captivity. In contrast, the many fish living in the wild have received considerably less attention, especially in relation to naturally occurring causes, such as parasitism, predation attempts, and intra-specific conflict. Here, I attempt to quantify the prevalence of injuries and deformities in wild fish by conducting a targeted review of six relevant journals, and suggest areas where future research would be particularly useful. The results indicate that both anthropogenic and non-anthropogenic factors can cause injuries and deformities in wild fish, and that many of the focal species (i.e. the species studied in the reviewed papers) are closely related. The average prevalence of injuries and/or deformities was 15% across studies. Despite the existence of potential confounding factors (e.g. a selection bias in terms of focal populations and species), these results highlight the potential importance of injuries and deformities as determinants and indicators of fish welfare in the wild.